Magnetic recording elements consist basically of a carrier or a substrate on which is coated an adherent layer of finely divided magnetic pigment particles which are dispersed in a matrix of polymeric binder.
The recording element is used for recording audio and/or video information. The element may take the form of a plate or disc, but it preferably has the form of a tape.
The carrier is usually manufactured from a synthetic resin, for example, polyester or polyvinyl chloride, but it may also consist of such materials as paper or metal.
The magnetic pigment comprises the usual ferromagnetic particles, for example, Fe particles, CrO.sub.2 particles or alpha-Fe.sub.2 O.sub.3 particles which optionally are doped with other elements such as Co, Ni, Zn, Sn or Ti.
In order to be able to readily disperse the pigment particles evenly throughout the binder, to prevent the formation of agglomerates of particles as well as to break up existing agglomerates, dispersing agents are used.
During operation, the tape is moved past the record and replay head of the record and replay device, which is better known as a recorder. If the tape has been out of use or stored for a while, the mechanical and magnetic properties of the tape are found to have changed. In particular, the frictional resistance of the tape sliding over the head has changed, which results in a different tape transport behavior. Applicants have found that the above-mentioned change is caused, at least in part, by the dispersing agent used in the magnetic coating.
A dispersing agent is used in order to be able to readily disperse the generally needle-shaped pigment particles having a length of about 0.1-1 .mu.m in the binder and thus to preclude the formation of agglomerate of particles or decompose them. Various dispersing agents are known for this purpose, such as lecithin, esters of phosphoric acid and alcohols which may be ethoxylated and alkylarylsulfonic acids. Dispersing agents are surface-active substances which have a surface-active, polar group in the molecule, such as a sulfonic acid group, a carboxyl group or a hydroxyl group as well as a nonpolar group, such as an aliphatic hydrocarbon radical with 12-20 carbon atoms. The polar group of the dispersing agent molecule adheres to the polar surface of the pigment particle. The nonpolar groups of the molecules of the dispersing agent so adsorbed on the surface of the pigment particle screen the said particle and preclude interaction with other pigment particles.
Applicants have found, in particular, that the above-mentioned change in the mechanical and magnetic properties of the recording element is caused by the fact that the dispersing agent used does not exhibit a uniform surface activity. The frequently used dispersing agent, lecithin, for example, contains a large number of components which have a very different surface activity. Within the large set of lecithin components, there is always one or more than one component which adsorbs at the special surface of the particle to be dispersed. A disadvantage is that the other components are loose in the system. The use of such a dispersing agent in the magnetic coating of recording elements has for its result that the nonadsorbed components of the dispersing agent soften the binder matrix and/or diffuse to the surface of the magnetic coating. This adversely affects, in particular, the mechanical properties of the recording element, particularly the frictional properties. Other disadvantages are a considerable soiling of the head and a sticky surface making layers stick together.
Another dispersing agent which is often used is based on esters of phosphoric acid and alcohols. Also in this case there are big differences in surface activity due to the fact that several components are present which each have a different surface activity. These components are, for example, free phosphoric acid, mono-, di- and tri-esters as well as the free alcohol. The presence of many components in a dispersing agent can often be attributed to the preparation process, which causes the introduction of impurities which cannot, or only with difficulty, be removed.
Therefore, there remains a significant need for a recording element having improved mechanical and magnetic properties. In particular, there is a need for a magnetic recording element, especially magnetic tape, having improved tape transport behavior, ergo one which contains a dispersing agent having more uniform surface activity.